In olefin epoxidation an olefin is reacted with oxygen to form an olefin epoxide, using a silver-based catalyst. The olefin oxide may be reacted with water, an alcohol or an amine to form a 1,2-diol, a 1,2-diol ether or an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers and alkanolamines may be produced in a multi-step process comprising olefin epoxidation and converting the formed olefin oxide with water, an alcohol or an amine.
Conventional silver-based catalysts have provided the olefin oxide notoriously in a low selectivity. For example, when using a conventional catalyst, the selectivity towards ethylene oxide, expressed as a fraction of the ethylene converted, does not reach values above the 6/7 or 85.7 mole-% limit. Therefore, this limit has long been considered to be the theoretically maximal selectivity of this reaction, based on the stoichiometry of the reaction equation7C2H4+6O2=>6C2H4O+2CO2+2H2O,cf. Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd ed., Vol. 9, 1980, p. 445.
Further, the catalysts are subject to an aging-related performance decline during normal operation. The aging manifests itself by a reduction in the activity of the catalyst. Usually, when a reduction in activity of the catalyst is manifest, the reaction temperature is increased in order to compensate for the reduction in activity. The reaction temperature may be increased until it becomes undesirably high, at which point in time the catalyst is deemed to be at the end of its lifetime and would need to be exchanged.
Over the years much effort has been devoted to improving epoxidation catalysts in their performance, for example in respect of their initial activity and selectivity, and in respect of their stability performance, that is their resistance against the aging-related performance decline. Solutions have been found in improved compositions of the catalysts, and, in other instances, solutions have been found in improved processes of preparing the catalysts.
Modern silver-based catalysts are highly selective towards olefin oxide production. When using the modern catalysts in the epoxidation of ethylene the selectivity towards ethylene oxide can reach values above the 6/7 or 85.7 mole-% limit referred to hereinbefore. Such high-selectivity catalysts may comprise as their active components silver, and one or more high-selectivity dopants, such as components comprising rhenium, tungsten, chromium or molybdenum. High-selectivity catalysts are disclosed, for example, in U.S. Pat. No. 4,761,394 and U.S. Pat. No. 4,766,105.
In respect of improved processes, U.S. Pat. No. 6,368,998, for example, shows that washing the support with water, or first with aqueous base and then with water, prior to the deposition of silver, leads to epoxidation catalysts which have improved initial performance properties. A further improvement can be achieved by depositing silver by impregnating the support with a silver containing impregnation solution which has a higher pH than conventional, for example, having a measured pH of 13.2 or 13.6 by the presence therein of additional base in the form of hydroxide.
In particular the high-selectivity catalysts are subject to an aging-related performance decline during normal operation and they tend to be exchanged more frequently than the conventional catalysts. It goes without saying that from an economical point of view it is highly desirable to improve the initial performance and the lifetime of high-selectivity catalysts as much as possible.